GRADING AND SCREENING
stroke. For fine screening and short strokes the head motions employed on ore-concentrating tables will give a satisfactory advance and the principles employed in these mechanisms can be used for greater strokes. A bumping device can also be used but has the disadvantage that it sets up destructive shocks in the screen frame and supports. If no differential motion is given a flat screen it must be fairly highly inclined or the capacity will be small. Inclinations of a few inches per foot give no results. On most material a good effect is not produced until an inclination of about 36 deg. is reached unless the motion is very strong or lively or very smooth material such as coal is being screened.
While not absolutely essential, balancing of flat shaking screens with high speeds overcomes destructive vibration. The condition for perfect counterbalance is that the momentum® of the screen and counter-balance and the screen must be equal. Another mode of counterbalancing is to employ pairs of screens employed in making
FIG. 14.—Bolter fly-wheel.
gradings and have the pairs move in opposition. This is commonly done with grain cleaning separators. In the Coxe and other movable bar grizzlies one set of bars moves forward as the other moves back giving a balancing effect. These bars run
FIG. 15.—Gyratory bolter. '
so slowly that balance is not needed, the transport!ve effect being the one sought for.
Balancing of gyratory bolters can be made quite simple. Figure 14 illustrates the flywheel producing gyration placed below one design of bolter at A and A (Fig. 15), and the means for balancing will be evident from the figures.
Interstitial Action in Mass Screening on Flat Shaking Screens.—In screening with hand screens it is noticed that if too deep a bod of material is placed on a screen, elimination of undersizc is very slow. This is due to packing in. the lower layers so that interstitial settlement of the fines cannot take place. With a depth of bed equal to five grains of size equal to the size of the screen aperture the packing begins to be serious. On a power screen the packing is not so bad since there is not the confinement of the material as there is in a heavily loaded hand